Formation mechanisms of quartz veins in orogenic gold deposits: Insights from Grass Valley, California, USA

The orogenic gold veins of Grass Valley, California, USA, compose the historically richest lode gold district in the North American Cordillera. Petrographically, the veins exhibit a range of primary textural relationships allowing the reconstruction of the paragenetic sequence of mineral formation. Two generations of quartz are distinguished by optical cathodoluminescence microscopy and fluid inclusion petrography. Early quartz formed under conditions of variable pressures, ranging from supralithostatic to sublithostatic, whereas late quartz formed entirely at hydrostatic conditions. Subsequent sulfide formation occurred from fluids increasingly buffered by the host rocks. Late growth zones in pyrite are enriched with elements derived from the local host rock and contain abundant gold inclusions. Gold was never observed encapsulated by the quartz, rather it resides along grain boundaries and fractures in the quartz. The results of this study underscore the importance of pressure fluctuations during quartz vein formation but suggest that the bulk of the gold in these orogenic systems was introduced late in the paragenesis when fluid flow occurred at hydrostatic conditions.